Is that callout still scratchy in the next game started (after the board has "warmed up")?

And what do you mean by "scratchy"? The old-school compression used means none of them are super clear - think lo-fi tape or landline phone. Compared to today's tech, some might describe it as "scratchy". That callout on mine seems a little harsher (if I wouldn't say "scratchy") than the others, but I just figured the high-pitched female voice didn't take as well to the compression. The "Ooh, Millions!" callout is similarly sub-par. But neither is offensively so, if that makes sense.

Well if it's consistently bad we can rule out it suffering as the literal first words spoken as the board is warming up. So if it's truly distinctly bad compared to the others, a new ROM might be worth trying. As you describe it, no, it shouldn't be that bad. Mine is perfectly audible, just a little harsh in its compressed tone (just like "Ooh Millions", with same voice and relative clarity).

You might reseat the ribbon cable first, just to try... I once experienced flaky certain sounds on my Bad Cats that ultimately traced to a cable, so it's possible. But I wouldn't necessarily hold my breath on a cable since the callout does actually play, and a bad ribbon line should affect more than one sound. As you guess, bit rot could cause bad quality solely on the affected data.

In pursuit of fidelity I would probably recap the board too as part of an overhaul. I greatly improved the quality of a Diner's sound package that way but all the sounds & callouts were all crappy, not just one.